Once a rosebush has the Black Spot fungus, it will not ever go away from the leaves it is on. However once killed, the new foliage will be free of it. My go to fungicide to stop a nasty Black Spot fungus attack in its tracks is a fungicide called Mancozeb. You can get it at Amazon.com. It does leave a yellowish powdery residue on the foliage, but that is part of how it works. Spray the rosebushes effected with this fungicide every 7 to 10 days for 4 to 5 sprayings. Two tablespoons of the Mancozeb to one gallon of water is the mix I use. Here is an article for you on this subject as well: https://www.gardeningknowhow.com/ornamental/flowers/roses/black-spot-roses.htm
Answered on May 24, 2020
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